Absorption spectroscopy

ABSTRACT

Gas in a sample region is analyzed in a nondispersive system for the presence of a particular gas of interest by cross-correlating the absorption spectra of the sample and a specimen of the gas of interest. Radiation is directed through the sample region and a filter for isolating a narrow band of frequencies within the absorption band of the gas of interest, to a radiation detector. The radiation is passed through cells that provide the total energy transmitted at the line or lines of interest and a reference energy transmitted by a region adjacent to the lines of interest and unaffected by changes in absorption in the sample region. In one embodiment this is accomplished by passing the radiation from the filter in rapid succession through three cells typically containing, respectively, an inert gas, the gas of interest at a pressure of about one atmosphere and the gas of interest at approximately two atmospheres pressure, to produce three signals which are combined by a conventional electronic system to give the fractional absorption of the gas of interest in the sample region. an interferometer containing in one leg, a cell containing the gas of interest at a pressure of about two atmospheres, may be located in the radiation path between the cells and the detector.

United States Patent [1 1 Blau, Jr.

[ Mar. 27, 1973 [54] ABSORPTION SPECTROSCOPY [75] Inventor: Henry H.Blau, Jr., Wayland, Mass.

[73] Assignee: Environmental Research 8:

Technology, Inc., Lexington, Mass.

[22] Filed: Mar. 18, 1971 [21] Appl. No.: 125,615

[52] U.S. Cl. ..250/43.5 R, 356/51 Goody, R. M. Cross-CorrelatingSpectrometer, J. Optical Soc of America, 58-900 (July 1968) PrimaryExaminer-Ronald L. Wibert Assistant Examiner-V. P. McGraw Attorney-Schiller & Pandiscio [57] ABSTRACT Gas in a sample region is analyzed ina nondispersive system for the presence of a particular gas of interestby cross-correlating the absorption spectra of the sample and a specimenof the gas of interest. Radiation is directed through the sample regionand a filter for isolating a narrow band of frequencies within theabsorption band of the gas of interest, to a radiation detector. Theradiation is passed through cells that provide the total energytransmitted at the line or lines of interest and a reference energytransmitted by a region adjacent to the lines of interest and unaffectedby changes in absorption in the sample region. In one embodiment this isaccomplished by passing the radiation from the filter in rapidsuccession through three cells typically containing, respectively, aninert gas, the gas of interest at a pressure of about one atmosphere andthe gas of interest at approximately two atmospheres pressure, toproduce three signals which are combined by a conventional electronicsystem to give the fractional absorption of the gas of interest in thesample region. an interferometer containing in one leg, a cellcontaining the gas of interest at a pressure of about two atmospheres,may be located in the radiation path between the cells and the detector.

20 Claims, 11 Drawing Figures ABSORPTION SPECTROSCOPY The presentinvention is concerned with spectroscopic analysis of gases particularlyby cross-correlation of the spectra of a specimen of the gas of interestand a sample.

Gas analyzers of the type with which the present invention are concernedfind utility in a number of different fields including, for example, airpollution monitoring, the analysis of gases passing through flues orstacks in connection with combustion control, as well as air pollutantcontrol, in such disparate fields as planetary, astronomy andmeteorology.

The operation of cross-correlating, spectroscopic systems are based onthe fact that most gases have a characteristic absorption spectrumcomprising a large number of narrow, discrete spectral lines. Thepresence and quantity of a particular gas in a sample region, such asthe ambient atmosphere, a flue, stack, or a contained sample, can bedetermined by measuring the cross-correlation between absorption over anarrow spectral region in a specimen of the gas of interest and a gassample. For many gases of interest, useful absorptionspectra can be mostreadily found in the infrared, but in some cases also in the visible andultraviolet spectral regions.

Dispersive systems involving the correlation between dispersed spectrahave been employed. However, since spectral features can be very narrow,practical embodiments usually suffer from the disadvantages attendantupon performing correlations with less than an optimum spectralresolution and from optical inefficiencies necessarily associated withuse of narrow slits and collimated light beams.

Nondispersive gas analyzers have been developed, typically incorporatinga filter isolating an appropriate narrow spectral region, a selectivechopper consisting essentially of a gas cell containing the gas ofinterest, and an identical cell evacuated or filled with a transparentgas so arranged that the cell containing the absorbing gas and thesecond cell are alternately in the radiation beam. Such a systemselectively chops the spectral lines of the gas of interest and themagnitude of the the signal developed by the detector depends ontransparency of the gas of interest at the spectral lines. The accuracyand sensitivity of such a system is limited by the impossibility ofperfectly balancing the transmissions of the two cells outside of thespectral regions where the gas absorbs, and also due to interferencesfrom the presence of absorbing gases with overlapping spectra or falsesignals produced by source fluctuations or any other intensityfluctuations more rapid than the rate at which the cells are alternatelyin the beam.

A nondispersive gas analyzer has been proposed 1. Goody, R. M.,Cross-Correlating Spectrometer, .I. Optical Soc. of America, 58,900(July 1968)., incorporating a nondispersive optical instrument thattransmits radiation only at the centers of spectral lines. A typicaldevice of this type is a Michelson interferometer with a cell in one armcontaining a specimen of the gas of interest in combination with afilter for isolating a relatively narrow spectral region. If theinterferometer is balanced to produce destructive recombination of allfrequencies in the spectral region of interest, one may detect thepresence of the gas in a sample region through which entering radiationis passed. The accuracy and sensitivity of such a system is limited dueto the impossibility of perfectly balancing the interferometer toeliminate leakage, and due also to the above-mentioned interferencesresulting from scattering or the presence of other absorbing gases withoverlapping spectra.

Objects of the present invention are: to provide a novel and improvedgas analysis system somewhat.

analagous in operation to the foregoing correlation spectrometers bututilizing absorption by the gas of interest and a reference signalderived from transmission of adjacent frequencies to the lines ofinterest to achieve correlations; to provide such a system in which thereference signal is derived from pressure broadening effects, and toprovide such a system in which one obtains greater sensitivity andselectivity by means of high optical efficiency, by optimizing effectivedispersion and hence the correlation function, by reducing interferencesdue to the presence of other absorbing gases with overlapping spectra,by reducing interferences due to scattering and absorption by smoke oraerosol particles in the beam, and by minimizing errors due to sourcefluctuations or unavoidable instrument instabilities.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed disclosure, and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be made to the following detailed description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a partly schematic, partly cross-sectional view of a gasanalysis system embodying the invention;

FIG. 2 is a schematic, cross-sectional view of a component of the systemof FIG. 1 taken along one line 2- 2 of FIG. 1;

FIGS. 3a and 3b provide a graphical representation of idealized signalsproduced by the gas analysis system of FIGS. 1 and 2, illustrating theoperation of this embodiment including the operations performed on andthe significance of the signals produced;

FIG. 4 is a view, similar to FIG. 1, illustrating another version of agas analysis system embodying the present invention;

FIG. 5 is a schematic, cross-sectional view of a component of FIG. 4,taken substantially along line 5-5 of FIG. 4;

FIG. 6 is a graphical representation, similar to FIG. 3, of the signalsproduced by the embodiment of FIGS. 4 and 5, illustrating the operationthereof;

FIG. 7 is a schematic cross-sectional view of another embodiment of thepresent invention;

FIG. 8 is a schematic, cross-sectional view of a component of FIG. 7,taken along line 8-8;

FIG. 9 is a graphical representation similar to FIG. 6, of idealizedsignal produced by the device of FIG. 7; and

FIG. 10 is a cell array useful in yet another variation of theinvention.

Generally, the nondispersive gas analysis system of the presentinvention involves the measurement of the fractional absorption ofradiation at a line or lines of interest in a sample zone containing thegas. This is accomplished by providing an optical filter which defines apass-band in which the lines of interest lie. Means are provided formeasuring the energy transmitted (F,) within or under the lines ofinterest for a unit time and also for measuring the energy transmitted(F in a spectral region very closely adjacent the lines of interest forthe unit of time, but unaffected by changes in absorption that mightoccur due to variations in the gas compositions in the sample zone orregion. Energy/unit time or flux F, thus constitutes a reference orbackground which remains substantially invariant with changes inabsorption. Means are then provided for determining the ratio (F ,Fwhich will be shown to be a measure of the fractional absorption.

A first embodiment of a specific nondispersive gas analysis systemembodying the invention is illustrated in FIGS. 1 and 2 of the drawings.This system utilizes pressure broadening to provide the referencesignals corresponding to transmission in narrow spectral intervalsadjacent to and at either side of an absorption line. The basiccomponents of the analyzer include an optical filter 21 capable ofisolating a band of frequencies within the absorption band of the gas tobe detected. For many gases of particular interest, this band lies inthe infrared region of the spectrum and in the preferred embodiment, maybe restricted to a wavelength region containing approximately I (orless) absorption lines of the subject gas. Any suitable narrow bandfilter such as an interference filter, grating spectrometer or the like,may be employed.

The analyzer may include its own radiation source (not shown) or receiveand operate in response to ambient radiation. It may also include aconventional optical system suchas a telescope 18, for causing a beam ofradiation passing through a sample region (which contains the gas to beanalyzed) to traverse filter 21. The gas to be analyzed or detected maybe in the surrounding atmosphere; it may be gas in motion through aparticular region such as a chimney or stack; or it can be a confinedsample of gas with controlled parameters including dimensions, pressure,temperature, concentration and the like.

A beam of light which has passed through the sample gas 20 and filter21, is then caused to traverse, alternately and in rapid sequence, aplurality of cells shown as an array 22; and is then directed by a lens23 to a radiant energy detector 24 such as a thermistor bolometer whichis sensitive to infrared radiation.

Cell array 22 is illustrated in FIG. 2 as comprising threeradiation-transmitting cells designated X, Y, and Z mounted for rotationin the path of radiation transmitted by the telescope through filter 18.Cell X is 'empty or filled with a gas N, such as nitrogen, which doesnot absorb in the spectral region of interest. Cell Y contains the gas Gof interest together with a nonabsorbent gas at a total pressure thatmight typically be about one atmosphere (P,) and at a partial pressureof G sufficient to achieve about 100 percent absorption at the center ofthe line of interest. The third cell, designated Z, contains gas G plusnonabsorbent gas and is at a total pressure typically of about 2atmospheres (P,). The partial pressures of the gases of interest G incell Z are adjusted to give an area under the absorption lines abouttwice the area under the absorption lines of the gas in the sample cellY.

The spectral signals reaching the detector and representing thetransmission of each of the three gas cells X, Y, and Z in the spectralregion defined by the interference filter are illustrated in FIG. 3A.The idealized transmission through cell X is shown as a signal 40 ashaving peak amplitude h, and a dip 42 to a smaller amplitude h,resulting from absorption by gas G at a sample absorption wavelength. Inpractice, many lines will be present in the pass-band of the filter, butonly one is shown for illustration. With cell Y in the beam,

peak amplitude remains at h but the transmission at the sampleabsorption band is reduced to near zero as shown by dip 43 because ofthe complete absorption at this wavelength by the gas G in cell Y. Thetransmission signal due to cell Z is similar to signal through cell Yexcept that the absorption line 43 pressure broadened, as previouslyindicated, to approximately twice the width of the comparable line ofthe gas in cell Y.

FIG. 3A also illustrates the spectral characteristics of the differencesignals (X -Y) and (Y-Z) which are either computed by hand or determinedin known electronic apparatus as will be described later. The former(X-Y) is centered on the line of interest and its envelope encloses atotal energy flux of F, defined by the area under the peak. Similarly,the difference signal used as a reference (Y-Z) is formed of two smallerpeaks lying to either side of the line of interest and each enclosingidentical energies summing to F, the total area under both peaks.

To illustrate one way of obtaining the fractional absorption of gas G inthe sample region, the pressure P, and P, and the partial pressure ofgas G in cells Y AND Z, respectively, are adjusted so that thedifference signals (X-Y) and (Y-Z) are equal when there is no absorptionby gas G in the sample region (i.e., F, F,).

Now if there is absorption due to the presence of gas G in region 20,then the energy flux F, will change by AF, but F, remains the same. Onecan then define transmission Tas the ratio:

Thus, if there is no absorption F, F, as a precondition and T= l. Thefractional absorption Q can thus be Y (X-Y The fractional absorptioncan, of course, still be obtained if A, is not exactly equal to the sumof A, and A, although the algebra will be somewhat different.

It will be apparent that the fractional absorption, which can be relatedto the quantity of gas G in the sample zone, may now be computed. Animportant feature of the invention is that the ratio I (FE Pla n I (Y-Z)is not only proportional to the transmission of gas G in the sampleregion, but is relatively independent of longterm radiation sourcefluctuations and interferences resulting from particulate scattering andabsorption by gases with overlapping spectral features. Thisindependence from interferences is very important in variousapplications such as air pollution monitoring in which the sample regionis ambient air or in stacks or flues in which the presence of otherspecies resulting in absorption and scattering losses, is not subject tocontrol. The relative importance of absorption and scattering losses dueto particulate matter in the sample region is minimized because thequantities (X-Y) and (Y-Z) are affected almost identically. This will betrue even though the absorption or scattering has some wavelengthdependence because the error depends on the curvature of the attenuationversus wavelength curve and in most instances, this can be expected tobe very small. The frequency at which the gas cells X, Y and Z aresequentially interposed in the radiation beam will have an effect on theinfluence of source fluctuations. If the frequency of interposition ofcells X, Y, and Z is high enough; i.e., it should be at least greaterthan the frequency of source fluctuations, then the effects thereof onsignals X, Y, and Z will cancel out.

Use of the three cells X, Y, and Z as described above permits comparingtransmission at or near line centers to transmission in two narrowspectral intervals adjacent to and either side of each absorption line.The location and width and to some degree shape of the two intervalsadjacent to each line can be varied in a predictable way by varying thetotal pressure and partial pressures of the gases in cells Y and Z. In alike manner,

the width and amplitude of the spectral signals (X- Y) in FIG. Acentered at the absorption line centers can be varied by controlling thetotal and partial pressures of gases in cell Y. Manipulation of (X-l)and (Y-Z) in this way permits minimizing interferences and optimizingsignal modulation for a given set of circumstances. For example, use ofvery narrow regions in (X-Y) and (Y- Z) by use of low pressures whichreduce broadening effects will help reduce interferences fromoverlapping absorption bands at the expense of reducing signal but maybe useful in producing a larger signal to background interference ratio.

Operation of the gas analyzer requires that H in X, Y, and Z beprecisely the same during one cycle of rotation of the cells. If theinstrument is operated as a remote sensor, and the field of view isscanned rapidly enough, this will not be the case and H variations willbe encountered, and must be accounted for.

Thus, an a preferred form, in instrument embodying the inventionincludes a reference channel formed by introducing a beam splitter suchas partially reflecting partially transmitting mirror 25 into the pathof radiation coming from cell array 22. In the path of radiationreflected by mirror 25 is reference cell 26 which contains the same gasmixture at the same total and partial pressures in cell Z. Radiationtraversing cell 26 then is focused by lens 27 into radiant energydetector 28 which converts the radiant energy into electrical signals.The spectral signals reaching the reference detector with X, Y, and Z inthe beam are respectively shown in FIG. 33 as X and Y,, and 2,.Differences in b, that occur with time are shown as A h in the latterFigure and will produce differences in the reference signal levelrecorded with cells X, Y and Z sequentially in the beam. These signalscan be used to normalize the main channel signals corresponding to X, Yand Z. This can be accomplished, for example, by multiplying in knownelectronic devices the amplitude of signals X, Y, and Z respectively bythe inverse of the corresponding reference channel signals X and Y,, and2,, recorded simultaneously with signals X, Y and Z.

The reference channels signals can be used to account for sourcefluctuations, fluctuations produced by scattering or absorption due tosmoke or aerosol, or changes in H produced by scanning the field ofview. In addition, reference channel signals can be used to account forinstrument-induced errors. The discussion thus far has been for the casewhere the transmission of cells X, Y, and Z away from the absorptionlines due to gas G are identical or very nearly identical. While thiscan be accomplished by conventional optical means with a very highdegree of precision, perfect matching can never be achieved. Furtherchanges can occur in time, for example, due to dirt deposits on thecells. Such changes can obviously be accounted for by use of thereference channel signals as described above.

Note that detector 24 sees in sequence (although three detectors can beused in parallelfor simultaneous viewing) the signals X, Y, and Z. Thesesignals may be digitized and stored in timed manner, called from storageand summed algebraically to obtain the requisite difference values (X-Y)and (Y-Z) and then the required ratio set up and computed. A simpleanalog-to-digital converter and digital computer, all generally shown ascomputation system 29 and wellknown in the art, can readily carry outthese simple computations. Alternatively in the simplest form, system 29need only be a meter indicating the integrated values of F l and F andcomputation carried out by hand. Obviously, analog electronic orelectromechanical systems can also be used to form system 29.

Another embodiment of the gas analyzer of the invention is illustratedin FIGS. 4 and 5. This system requires a lesser degree of accuracy inthe measurement of the X, Y, and Z signals, and is similar to thatpreviously shown and described. It differs in that optical means fortransmitting radiation only at the center of a spectral region or bandare located between the array 22 of gas cells and detector 24. Anexample of such means is a Michelson interferometer shown simply asincluding beam splitter 30 and the usual pair of mirrors 31 and 32. Acell 33 containing gas G tobe detected together with an inert gas atabout the same total and partial gas pressure as in cell Z is located inone of the legs of the interferometer. Mirrors 31 and 32 of theinterferometer are adjusted to provide as nearly as possible a phasedifference for wavelengths or frequencies in the band-pass of filter 21.Thus, when properly balanced, the interferometer will transmit verylittle radiation except at wavelengths corresponding to absorption bythe gas G to bedetected where the balance is disturbed by the presenceof gas G (in the sample region).

In the embodiment shown in FIGS. 4 and 5, cell array 22 includesradiation-transmitting cells designated X, Y, and Z. Exactly as in theembodiment of FIG. 1, cell X is empty or filled with inert gas N; cell Ycontains at least the gas of interest G together with an inert gas at atotal pressure typically of about one atmosphere (P and the third cell Zcontains gas G plus an inert gas at a total pressure typically of about2 atmospheres (P Gas G is at sufficient partial pressures in cells Y andZ to effect approximately complete absorption at line centers, and thetotal and partial gas pressures are such that the area under the linesin Z is twice the area under the lines in Y. Rather than rotate thearray 22 of gasfilled cells to alternately interpose the cells in theradiation beam traversing sample region and filter 18, it may bepreferable to hold the cells stationary and move an optical system toproduce radiation transmission through successive cells. In such asystem illustrated in FIGS. 4 and 5, the cell array designated 22including cells X, Y, and Z, is held stationary. A pair of periscopes(e.g., prisms) 36 and 37 are mounted on a shaft 38 mounted forsimultaneous rotation (as shown by the curved arrow) about the axis ofthe radiation beam for successively directing the path of the radiationbeam through cells X, Y, and Z.

As periscopes 36 and 37 are rotated to interpose cells X, Y, and Z, inthe radiation beam, detector 24 will receive the typical, idealizedspectral signals X and Y and Z,, illustrated in FIG. 6 together with thedifference signals (X-Y), and (Y-Z),. It will be noted from FIG. 6 thattransmission with cell X in the beam is largelyconfined to the regionrepresented by peak 60 around where the pressure broadened spectral lineof the gas of interest G absorbs. This spectral region is approximatelytwice as wide as the line width due to the presence gas G in the sampleregion at a pressure of one atmosphere (P However, because theinterferometer cannot be balanced perfectly over the spectral regionemployed, the peak 60 is superimposed on a weak background signal orpedestal 62. Absorption by gas G in the sample region is represented bya dip 64 to a smaller amplitude h When cell Y is interposed in the beam,the signal produced is identical to signal X except that transmission atthe line center for gas G is reduced to zero as shown at 66 because ofthe absorption of gas G in cell Y. The total and partial pressures ofgases in cell Z is such that the absorption line is pressure broadenedto about the same width as the line of gas G in the cell 33 in theinterferometer so that the signal Z contains primarily only leakageradiation. The amplitude peak 60 of signals X and Y is replaced by a dipbecause all radiation at this frequency is absorbed by passing throughthe gas G at pressure P, in cell Z.

It now can be seen that the only difference between the X, Y, and Zsignals illustrated in FIG. 3 and the X,, Y,, and Z, signals of FIG. 6,is the presence of the background signal 62 so that the differencesignals (X Y) and (y-z) produced by each system represent the samevalues and once again, the fractional absorption due to the presence ofthe gas of interest G will be given by the ratio:

A reference channel arrangement similar to that described in connectionwith FIG. 1 can also be used.

It will be seen from the foregoing that the gas analysis system of theinvention achieves independence from source fluctuations by the rapid,sequential interposition, in the radiation beam directed through thesample region to the detector, of three radiation transmitting cellscontaining, respectively, an inert gas and the gas of interest atdifferent pressures. Substantially increased accuracy, sensitivity andsignal-to-interference ratio are achieved by pressure broadening and byeliminating spurious and background signals resulting from leakage andinterference from other absorbing or scattering species such asparticulate matter, smoke, water vapor, and noncondensible gases. Theabsorption by the gas of interest is caused to produce the desiredcorrelation so that the effective dispersion of the system is optimized.The requirements for accuracy in signal detection in order to produceprecise results is reduced by interposing between the three cells andthe detector, an interferometer having in one leg, a cell containing thegas of interest at a pressure of about two atmospheres.

In place of gas G at pressure P in cell Z for the system with nointerferometer such as is shown in FIG. 1, a second gas S not present inthe sample zone but with spectral lines in the spectral regiontransmitted by filter 18 can be used. As shown in FIG. 10, minimaloverlap of spectral lines of G and S is desirable but not critical. Thetotal pressure in cell Z is' made approximately the same as in cell Y;i.e., P The partial pressures of S and G and/or total pressures areadjusted initially so that signals (X-Y) and (X-Z) are equal with no gasG present in the sample space and the fractional absorption of gas G isthen determined from the relationship or by simple algebra (Y-Z)/(X-Z).

In the latter case the numerator will be zero with no absorption by gasG in the sample zone and will take on a maximum value equal to X-Z whenthere is percent absorption by G in the sample zone. This suggestsanother embodiment in which cells Y and Z are alternately in the beam toproduce Y-Z. A third cell designated Y identical to Y with I00 percentabsorption by G would be periodically introduced in the beam in serieswith Y and Z to produce a signal designated Y'-Z and the fractionalabsorption determined from For the system with the interferometer suchas is shown in FIG. 4, cell 30 in the interferometer can contain both Gand S at a total pressure of about P, with the partial pressures and Gand S adjusted to give approximately equal transmissions of energy inthe spectral regions occupied by lines of G and S respectively. Thearrangement of X, Y and Z is as described above. In either case, itshould be observed that (X-Z) is a reference signal derived from aregion directly adjacent the line of interest but which shows novariance with respect to changes ofG in the sample zone.

As with the pressure broadened system the total pressure and partialpressures of S and G can be ad-' justed to optimize signal tointerference or background ratios for a given situation. Because of theselective shopping arrangement, some overlap of lines of S and G doesnot significantly influence results.

In another arrangement, the gas cells as shown in FIGS. 7 and 8 includecell V evacuated or filled with a transparent gas, cell Y containing gasG plus an inert gas at a total pressure P and cell S filled identicallyto cell Y. As shown in FIG. 7, cells V and S are each followed byprecision variable radiation broadband attenuators 50 and 52; i.e., adevice for uniformally alternating radiation intensity over a wide rangeof spectral frequencies. The remainder of the system can be the same aseither the configuration of FIG. 1 or FIG. 4 and, hence, like referencenumerals indicate like parts.

To operate the system of FIGS. 7 and 8 with no gas G in the sample zone20, attenuator 50 following cell V is set so that the total energies perunit time or flux in the signals passed by each of cells V and Y areprecisely equal. Attenuator 50 can simply be a plane transparent platewhich is set to refract and reflect respective portions of an incominglight beam. Now with gas G in zone 20, the idealized spectral signalfrom cell V as shown in FIG. 9 at V exhibits the usual partialabsorption dips as at 54. The signal from cell Y as shown in FIG. 9 asY; remains unaffected by the presence of gas G in zone because of itscomplete absorption of the line of interest due to the presence of gas Gin cell Y. The computed difference signal (V- Y shown in FIG. 9 issomewhat different than the than the previous signals such as (X-Y).This is because the precondition (that when no absorption occurs insample zone 20, V=Y effectively insures that (V-Y is,,in effect, thesame as F Now because cells S and Y are identical save for the outputattenuator 52 associated with the former, the difference signal (Y is areference signal which in effect is F Hence. the ratio (Hoard) a easuref F1)/F2|1 or the same fractional absorption ratio as determined by theother embodiments.

It will be seen that, alternatively, a variation of the embodiment ofFIG. 7 can be made employing but two cells, V and Y, and periodicallyintroducing attenuator 52 behind cell Y to obtain the signal S.

The systems using respectively pressure broadening or the second gas Swith overlapping spectral features are virtually independent of effectsdue to changes in the pressures in the cells that would be produced byambient temperature changes. This is true regardless of whether thelines due to gas G in the cells X, Y and Z are in the linear or squareroot regions. For the system just described, independence fromtemperature induced pressure changes occurs only for lines in the linearregion.

This requires further explanation. Consider a single spectral line. Thetotal pressure can affect the area encompassed by the single line. Whenabsorption at the line center is nearly complete, the area varies withthe square root of the total pressure. Thus, for a fixed amount of gasG, the area will change as P or P, changes. When absorption at linecenter is quite small, there is no pressure dependence.

Where there is no pressure effect, there is no problem with any of theconfigurations described. Where there is a pressure effect, the systemsof FIGS. 1 and 4 are not sensitive to the pressureinduced changesbecause the effect is the same for the spectral line Lil (signal X Y)and for the comparison lines (Y-Z) in the pressure broadening case and(X-Z) for the case with gas S. Since we take a ratio, the pressureeffect cancels out. This is a real advantage since it gives more freedomin choice of total and partial pressures for optimizing asignal-to-detector noise and signal-to-interference levels.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. In an optical, nondispersive absorption analyzer having a filter forpassing radiation in a relatively narrow spectral region includingabsorption lines of a material of interest, a radiation detectorsensitive in said region and means for directing a beam of radiationwhich has passed through a sample region containing material to beanalyzed and thence through said filter to said detector, theimprovement comprising, in combination:

means for determining total energy flux F transmitted to said detector,within said lines of interest and for determining total energy flux Ftransmitted to said detector in regions immediately adjacent to saidlines of interest and substantially invariant with changes in absorptionat said lines of interest by said material in said sample region;

said means for determining said total energies comprising:

a first light-transmitting cell capable of substantially nonabsorptivelytransmitting radiation at said lines of interest;

a second light-transmitting cell capable of substantially absorbing allof the radiation within only said lines of interest, and

a third light-transmitting cell capable of substantially absorbing allof the radiation with said lines of interest and only selected regionsimmediately adjacent to said lines of interest.

2. An absorption analyzer as defined in claim 4 wherein said material insaid sample region is a gas of interest, and

said first light-transmitting cell contains a gas capable ofsubstantially nonabsorptively transmitting radiation at said lines ofinterest;

said second light-transmitting cell contains said gas of interest at atleast a partial pressure sufficient to effect substantially completeabsorption of said lines of interest in radiation passing through saidsecond cell; and

said third light-transmitting cell contains said gas of interest at apartial pressure substantially greater than the partial pressure of saidgas in said second cell.

3. An absorption analyzer as defined in claim 5 wherein said gas ofinterest within said third cell is at a partial pressure such that theabsorption lines of interest are broadened to substantially twice thewidth of the corresponding absorption lines of interest of said gaswithin the sample region and said second cell.

4. An absorption analyzer as defined in claim 5 wherein said first cellcontains a gas which is nonabsorbent with respect to said lines ofinterest.

5. An absorption analyzer as defined in claim 1 including means forcausing said beam of radiation to pass through said first, second andthird cells so as to cause said detector to produce first, second andthird transmission signals respectively.

6. An absorption analyzer as defined in claim 5 wherein the means forcausing said beam to pass through said cells include means for rotatingan array of said cells as a unit to position each of said cellssuccessively in the path of said beam.

7. An absorption analyzer as defined in claim 5 wherein said first,second, and third cells are stationary and the last mentioned meansinclude movable lightdeviating means.

8. An absorption analyzer as defined in claim 5 wherein said means forcausing said beam to pass through said cells cause said cell and beam tomove with respect to each other at a frequency substantially exceedingthe frequency of substantial fluctuations in the intensity of saidsource.

9. An absorption analyzer as defined in claim 1 further includinginterference means for transmitting substantially only at the center ofa spectral line corresponding to a predetermined absorption line ofinterest, said interference means being located to transmit said beamfrom said cells to said detector and including a fourthlight-transmitting cell capable of substantially absorbing all of theradiation with said predetermined absorption line, and means for passingat least a part of said beam through said fourth cell.

10. An absorption analyzer as defined in claim 9 wherein saidinterference means include an interferometer and said fourth cell islocated in one leg of said interferometer.

11. An absorption gas analyzer as defined in claim 9 wherein saidmaterial in said sample region is a gas of interest, and said fourthcell, and said third cell, both contain said gas of interest atsubstantially the same pressure.

12. An absorption analyzer as defined in claim 2 further including aninterferometer for transmitting said beam from said cells to saiddetector and a fourth light transmitting cell containing said gas ofinterest located in a leg of said interferometer.

13. An absorption analyzer as defined in claim 12 wherein saidinterferometer is balanced to transmit relatively little radiationexcept at a wavelength corresponding to an absorption line of said gasof interest.

14. An absorption analyzer as defined in claim 12 wherein saidinterferometer is balanced to give destructive interference at thewavelengths at the center of the spectral region of interest.

15. An absorption analyzer as defined in claim 12 wherein said gas ofinterest in said fourth cell is at substantially the same pressure assaid gas of interest in said third cell.

16. An absorption analyzer as defined in claim 15 wherein said gas insaid third and fourth cells is at a pressure sufficient to broaden thewidth of said absorption line to substantially twice the absorption linewidth at the pressure of said gas of interest in said sample region andsaid second cell.

17. An absorption analyzer as defined in claim 2 further including meansfor so combining said signals as to obtain the fractional absorption ofthe gas of interest in the sample region as expressed by the ratiowherein said detector is adapted to generate signals X, Y, and Zcorresponding respectively to the total radiation flux transmittedrespectivelyby said first, second, and third cells from said sampleregion. v 18. An absorption analyzer as defined in claim 1 wherein saidmaterial in said sample region is a gas of interest;

said first cell contains a gas capable of substantially nonabsorptivelytransmitting radiation at said lines of interest; said second cellcontains the gas of interest at a partial pressure at least sufficientto effect substantially complete absorption of said lines of interest inradiation passing through said second cell; and said third cell containsanother gas having absorption characteristics in at least one regionclosely adjacent to a corresponding line of interest and being at apartial pressure approximately the partial pressure of the gas ofinterest in said second cell. 19. An absorption analyzer as defined inclaim 18 wherein said detector is adapted to generate signals X, Y, andZcorresponding respectively to the total radiation flux transmitted bysaid first, second, and third cells from said sample region, and

further including means for so combining said signals as to obtain thefractional absorption of the gas of interest in the sample region asexpressed by the ratio 20. An absorption analyzer as defined in claim 1wherein said material in said sample region is a gas of interest andwherein said means for determining said total energies comprises a firstlight-transmitting cell capable of substantially nonabsorptivelytransmitting radiation as said lines of interest;

a second light-transmitting cell containing said gas of interest,

means for selectively and uniformly attenuating, over substantially saidspectral region, radiation transmitted by said first and second cells,

said detector being adapted to generate signals V, Y

and S corresponding respectively to the total radiation flux transmittedby respectively a combination of said first cell and said attenuatingmeans, said second cell, and a combination of said second cell and saidattenuating means, and further including means for so combining saidsignals as to obtain the fractional absorption of the gas of interest inthe sample region as expressed by the ratio (V- Y)/( Y-S), theattenuation provided by said attenuating means being such that thecombination of said first cell and said attenuator passes the same fluxas said second cell along when no gas of interest is in said sampleregion.

t t t I l

1. In an optical, nondispersive absorption analyzer having a filter forpassing radiation in a relatively narrow spectral region includingabsorption lines of a material of interest, a radiation detectorsensitive in said region and means for directing a beam of radiationwhich has passed through a sample region containing material to beanalyzed and thence through said filter to said detector, theimprovement comprising, in combination: means for determining totalenergy flux F1 transmitted to said detector, within said lines ofinterest and for determining total energy flux F2 transmitted to saiddetector in regions immediately adjacent to said lines of interest andsubstantially invariant with changes in absorption at said lines ofinterest by said material in said sample region; said means fordetermining said total energies comprising: a first light-transmittingcell capable of substantially nonabsorptively transmitting radiation atsaid lines of interest; a second light-transmitting cell capable ofsubstantially absorbing all of the radiation within only said lines ofinterest, and a third light-transmitting cell capable of substantiallyabsorbing all of the radiation with said lines of interest and onlyselected regions immediately adjacent to said lines of interest.
 2. Anabsorption analyzer as defined in claim 4 wherein said material in saidsample region is a gas of interest, and said first light-transmittingcell contains a gas capable of substantially nonabsorptivelytransmitting radiation at said lines of interest; said secondlight-transmitting cell contains said gas of interest at at least apartial pressure sufficient to effect substantially complete absorptionof said lines of interest in radiation passing through said second cell;and said third light-transmitting cell contains said gas of interest ata partial pressure substantially greater than the partial pressure ofsaid gas in said second cell.
 3. An absorption analyzer as defined inclaim 5 wherein said gas of interest within said third cell is at apartial pressure such that the absorption lines of interest arebroadened to substantially twice the width of the correspondingabsorption lines of interest of said gas within the sample region andsaid second cell.
 4. An absorption analyzer as defined in claim 5wherein said first cell contains a gas which is nonabsorbent withrespect to said lines of interest.
 5. An absorption analyzer as definedin claim 1 including means for causing said beam of radiation to passthrough said first, second and third cells so as to cause said detectorto produce first, second and third transmission signals respectively. 6.An absorption analyzer as defined in claim 5 wherein the means forcausing said beam to pass through said cells include means for rotatingan array of said cells as a unit to position each of said cellssuccessively in the path of said beam.
 7. An absorption analyzer asdefined in claim 5 wherein said first, second, and third cells arestationary and the last mentioned means include movable light-deviatingmeans.
 8. An absorption analyzer as defined in claim 5 wherein saidmeans for causing said beam to pass through said cells cause said celland beam to move with respect to each other at a frequency substantiallyexceeding the frequency of substantial fluctuations in the intensity ofsaid source.
 9. An absorption analyzer as defined in claim 1 furtherincluding interference means for transmitting substantially only at thecenter of a spectral line corresponding to a predetermined absorptionline of interest, said interference means being located to transmit saidbeam from said cells to said detector and including a fourthlight-transmitting cell capable of substantially absorbing all of theradiation with said predetermined absorption line, and means for passingat least a part of said beam through said fourth cell.
 10. An absorptionanalyzer as defined in claim 9 wherein said interference means includean interferometer and said fourth cell is located in one leg of saidinterferometer.
 11. An absorption gas analyzer as defined in claim 9wherein said material in said sample region is a gas of interest, andsaid fourth cell, and said third cell, both contain said gas of interestat substantially the same pressure.
 12. An absorption analyzer asdefined in claim 2 further including an interferometer for transmittingsaid beam from said cells to said detector and a fourth lighttransmitting cell containing said gas of interest located in a leg ofsaid interferometer.
 13. An absorption analyzer as defined in claim 12wherein said interferometer is balanced to transmit relatively littleradiation except at a wavelength corresponding to an absorption line ofsaid gas of interest.
 14. An absorption analyzer as defined in claim 12wherein said interferometer is balanced to give destructive interferenceat the wavelengths at the center of the spectral region of interest. 15.An absorption analyzer as defined in claim 12 wherein said gas ofinterest in said fourth cell is at substantially the same pressure assaid gas of interest in said third cell.
 16. An absorption analyzer asdefined in claim 15 wherein said gas in said third and fourth cells isat a pressure sufficient to broaden the width of said absorption line tosubstantially twice the absorption line width at the pressure of saidgas of interest in said sample region and said second cell.
 17. Anabsorption analyzer as defined in claim 2 further including means for socombining said signals as to obtain the fractional absorption of the gasof interest in the sample region as expressed by the ratio wherein saiddetector is adapted to generate signals X, Y, and Z correspondingrespectively to the total radiation flux transmitted respectively bysaid first, second, and third cells from said sample region.
 18. Anabsorption analyzer aS defined in claim 1 wherein said material in saidsample region is a gas of interest; said first cell contains a gascapable of substantially nonabsorptively transmitting radiation at saidlines of interest; said second cell contains the gas of interest at apartial pressure at least sufficient to effect substantially completeabsorption of said lines of interest in radiation passing through saidsecond cell; and said third cell contains another gas having absorptioncharacteristics in at least one region closely adjacent to acorresponding line of interest and being at a partial pressureapproximately the partial pressure of the gas of interest in said secondcell.
 19. An absorption analyzer as defined in claim 18 wherein saiddetector is adapted to generate signals X, Y, and Z correspondingrespectively to the total radiation flux transmitted by said first,second, and third cells from said sample region, and further includingmeans for so combining said signals as to obtain the fractionalabsorption of the gas of interest in the sample region as expressed bythe ratio
 20. An absorption analyzer as defined in claim 1 wherein saidmaterial in said sample region is a gas of interest and wherein saidmeans for determining said total energies comprises a firstlight-transmitting cell capable of substantially nonabsorptivelytransmitting radiation as said lines of interest; a secondlight-transmitting cell containing said gas of interest, means forselectively and uniformly attenuating, over substantially said spectralregion, radiation transmitted by said first and second cells, saiddetector being adapted to generate signals V, Y and S correspondingrespectively to the total radiation flux transmitted by respectively acombination of said first cell and said attenuating means, said secondcell, and a combination of said second cell and said attenuating means,and further including means for so combining said signals as to obtainthe fractional absorption of the gas of interest in the sample region asexpressed by the ratio (V-Y)/(Y-S), the attenuation provided by saidattenuating means being such that the combination of said first cell andsaid attenuator passes the same flux as said second cell along when nogas of interest is in said sample region.